In forestry or horticulture it is often required to deliver a preset quantity of fluid such as liquid herbicide or liquid fertiliser from a sprayer. In forestry or farm forestry for example, herbicide is spot sprayed to cover typically 1m.sup.2 of surrounding ground prior to planting a seedling, or alternatively once a seedling has been planted a shot of selective herbicide is sprayed over the seedling and surrounding ground after planting. A forestry worker may spray a large number of seedlings over the course of a day. It is necessary that a constant amount of herbicide is sprayed around each seedling. Herbicides are expensive and it is also potentially hazardous to use more than is required, but at the same time it is essential that enough herbicide is sprayed around each seedling to do the job. In horticulture it is also often necessary to deliver preset quantities of herbicides, or alternatively liquid fertiliser or the like on or around plants. Again, manually operated back sprayers are often used.
A back pack sprayer typically consists of a tank for the herbicide or fertilizer to be sprayed, which is carried on the back of the operator like a pack. There is also an outlet hose from the tank and a delivery gun used by the operator which is usually trigger operated. There is also a handle which the operator pumps to pressurise the sprayer so that when the operator pulls the trigger of the spray gun the fluid will be delivered as a spray. Delivering a constant quantity of the herbicide or fertiliser requires that the operator maintains a constant pressure and holds the trigger open for a constant period in each operation, which is obviously only a rough estimation.
In another type of sprayer, instead of the spray tank being pressurised the hand held spray gun is based on a modified drench gun and has a pump-action trigger which when squeezed causes spray to be ejected. The squeezing action on each operation pumps the spray, so that when the device is used repeatedly the operators hand quickly becomes tired.
Back pack sprayers incorporating an electric solenoid valve and battery are known. The electric solenoid delivers a constant quantity of spray on each operation, but these sprayers require a power source to operate and are dependent on battery life.